Peripheral Neuropathic Pain
Chronic pain may be divided into two major categories: “nociceptive” pain (caused by inflamed or damaged tissue that activates specialized pain sensors called nociceptors), and “neuropathic” pain (caused by damage to or malfunction of the nervous system). Neuropathic pain may either be “peripheral”, which originates in the peripheral nervous system, or “central”, which originates in the brain or spinal cord.
Peripheral neuropathic pain is a chronic condition that is very common in clinical practice. Peripheral neuropathic pain can have many underlying causes, including diabetes, alcoholism, vitamin deficiencies, injury, toxic reactions to prescribed drugs, infectious diseases, malignancies, etc.
Several mechanisms contribute to the development and severity of neuropathic pain, which are very different from the mechanisms that cause nociceptive pain. As a result of nerve damage, several pathologic changes may occur including:                Impulse generation within the damaged nerve fiber        Nerve fiber to nerve fiber interactions        Failure of the normal nerve fiber inhibitory mechanisms        Plasticity—degeneration and regeneration of the injured nerve fibers that results in altered conductivity        
The pain associated with peripheral neuropathy may be severe and, unlike most pain that sufferers have previously experienced, it is characterized by allodynia (pain response from stimuli that do not normally provoke pain), hyperalgesia and in some cases, sensory loss. The pain is often described as “burning, stabbing, raw or sickening”. It can be constant or paroxysmal and with or without sensory impairment. The type and severity of pain or sensory loss depends on the underlying cause of the neuropathy. In chronic forms of neuropathic pain, symptoms begin subtly and progress slowly. Some people may have periods of relief followed by relapse. Others may reach a plateau stage where symptoms stay the same for many months or years. Many chronic neuropathies worsen over time. For the majority of patients with neuropathic pain, the pain will persist for life. Comorbidities such as depression, poor quality of life and employment and domestic issues are very common.
More than 100 types of peripheral neuropathy have been identified, each with its own symptoms and prognosis. In general, peripheral neuropathies are classified according to the type of damage to the nerves. Some forms of neuropathy involve damage to only one nerve and are called mononeuropathies. More frequently, however, multiple nerves are affected, called polyneuropathy.
Common types of peripheral neuropathic pain are depicted in Table 1:
TABLE 1Common Types of Peripheral Neuropathic PainAcute and chronic inflammatory demyelinating polyradiculoneuropathyAlcoholic polyneuropathyChemotherapy-induced polyneuropathyComplex regional pain syndromeEntrapment neuropathies (eg, carpal tunnel syndrome)HIV sensory neuropathy Iatrogenic neuralgias (eg, postmastectomy pain orpost-thoracotomy pain)Idiopathic sensory neuropathyNerve compression or infiltration by tumorNutritional deficiency-related neuropathiesPainful diabetic neuropathyPhantom limb painPost-herpetic neuralgiaPost-radiation plexopathyRadiculopathy (cervical, thoracic, or lumbosacral)Toxic exposure-related neuropathiesTic douloureux (trigeminal neuralgia)Post-traumatic neuralgia